In connection with the recovery of hydrocarbons from the earth, wellbores are generally drilled using a variety of different methods and equipment. According to one common method, a roller cone bit or fixed cutter bit is rotated against the subsurface formation to form the wellbore. The drill bit is rotated in the wellbore through the rotation of a drill string attached to the drill bit and/or by the rotary force imparted to the drill bit by a subsurface drilling motor powered by the flow of drilling fluid down through the drill string and through the drilling motor.
Downhole vibrations and shocks (referred to collectively and/or interchangeably herein as “shock loads”) are induced by interactions between the rotating bit and various types of hard rock and/or “sticky” earth formations at or near the floor of the wellbore. Shock loads induced at the drill bit are in turn transmitted to other components of the bottomhole assembly, as well as to the supporting drill string. Shock loads imparted on the drill string can diminish the life of its interconnected members by accelerating the process of fatigue. Additionally, excessive shock loads can cause spontaneous downhole equipment failure, wash-outs and a decrease in penetration rate.
Axial shock loads tend to cause a condition known as “bit bounce,” where the drill bit momentarily lifts up and loses contact with the floor of the wellbore. Bit bounce is known to cause acute damage to bit cutters and supporting bearings. Torsional shock loads are often caused by a phenomenon known as “stick-slip.” Stick-slip occurs when the drill bit stalls (e.g., drags or stops rotating completely) due to friction with the earth formations in the wellbore. When the drill bit stalls, typically, the attached drill string continues to turn, which can result in damage to the drill string and/or other components of the bottomhole assembly. Even if the operating torque applied through the drill string eventually succeeds in breaking the bit free of the formation, (i.e., overcoming the friction torque load on the bit resulting in a stall), the sudden release of the bit can cause it to rotate faster than the drill string. Stick-slip can cause problems in the operation of the drilling assembly and in the formation of the wellbore. In some cases, severe stick-slip can cause strong lateral vibrations in the drill string, which are also damaging.
Downhole shock loads are a major contributor to the failure of various components of the downhole equipment. Downhole shock loads may also cause damage to the wellbore itself (e.g., when lateral vibrations cause the drill string to contact the walls of the wellbore). Thus, mitigation of downhole shock loads is key to avoiding non-productive time and preventing equipment damage
Many of the features are exaggerated to better show the features, process steps, and results. Like reference numbers and designations in the various drawings indicate like elements.